Smoke detection and alarm systems incorporating smoke alarms are extensively used in domestic dwellings, motels, hotels, hospitals, old people's homes and other commercial premises. Such systems are of four main types all of which present difficulties or disadvantages as follows:
Type 1
A smoke detection and alarm system comprising of low cost battery powered self-contained stand-alone smoke alarms utilizing internal batteries, usually of the 9 V non-rechargeable type, for their operation. This type of smoke alarm has a very low quiescent current and at regular intervals, normally not exceeding 60 seconds, the smoke alarm enters into a self-test mode when the current is briefly increased to a value much above the quiescent current. The electronic circuitry of the smoke alarm detects whether the battery voltage in self-test mode is above a certain threshold value. If the battery voltage is detected to be below the threshold value, normally around seven and a half volts, the smoke alarm activates an internal circuit to produce a brief audible warning indicating a low or depleted battery requiring replacement.
The difficulties/disadvantages of Type 1 smoke alarms are:
(i) The smoke alarms are powered from only one source, their internal batteries. Should the battery of a smoke alarm be removed or disconnected, the smoke alarm becomes inoperative and often this condition cannot be detected until the smoke alarm is tested. This may result in quite a dangerous situation should a fire break out whilst the smoke alarm is inoperative. PA1 (ii) The 9 V battery is used to power the smoke alarm at all times and although Type 1 smoke alarms are designed to be very economical of batteries, the batteries only last about one year. PA1 (i) The constant monitoring of the smoke alarm stand-by battery results in a small but constant current drain which effectively reduces the battery life which, again, is around one year. PA1 (ii) The design of the smoke alarm, although offering a full battery back-up for times of mains failure, is complicated and costly due to the provision of safety features to avoid accidental and potentially lethal contact with live parts of the smoke alarm while the stand-by battery is being replaced. PA1 (iii) Because the smoke alarms are hard wired directly to the mains wiring system, the installation of the smoke alarms is specialised and is required to be carried out by a licensed electrician, thus adding to the overall installation cost of the system. PA1 switching the voltage supplied to the smoke alarms such that when the smoke alarms are in quiescent mode, the current drain is via the primary power supply, if available, and not via the stand-by battery power supply. PA1 providing a primary supply voltage which is slightly higher than the smoke alarm stand-by battery voltage; PA1 detecting if a current higher than the quiescent current is supplied by the primary power supply in accordance with whether any of the smoke alarms is in alarm or self-test mode, and PA1 lowering the primary supply voltage when a current higher than the quiescent current is detected. PA1 detecting when a current higher than the quiescent current is supplied by the primary power supply for a period of time exceeding the duration of the self-test mode, and PA1 raising the primary supply voltage to its original quiescent value when it is detected that a current higher than the quiescent current is supplied by the primary power supply for a period of time exceeding the duration of the self-test mode. PA1 detecting an alarm registered at any smoke alarm in the system, and PA1 supplying an alarm signal to the dedicated conductor whereby other smoke alarms in the system are activated. PA1 a primary power supply; PA1 a plurality of smoke alarms powered by the primary power supply, each smoke alarm having a stand-by battery power supply and being operable in a self-test mode, an alarm mode or a quiescent mode, the current drain in self-test and alarm modes exceeding the quiescent current drain, and PA1 switching means for switching the voltage supplied to the smoke alarms such that when the smoke alarms are in quiescent mode, the current drain is via the primary power supply, if available, and not via the stand-by battery power supply. PA1 means for providing a primary supply voltage which is slightly higher than the smoke alarm stand-by battery voltage; PA1 means for detecting if a current higher than the quiescent current is supplied by the primary power supply in accordance with whether any of the smoke alarms is in alarm or self-test mode, and PA1 means for lowering the primary supply voltage when a current higher than the quiescent current is detected. PA1 means for detecting when a current higher than the quiescent current is supplied by the primary power supply for a period of time exceeding the duration of the self-test mode, and PA1 means for raising the primary supply voltage to its original quiescent value when it is detected that a current higher than the quiescent current is supplied by the primary power supply for a period of time exceeding the duration of the self-test mode. PA1 (1) the reactive primary power supply comprises: PA1 (2) the smoke alarms comprise: PA1 (b1) means by which all the current required under quiescent condition, with primary power available, is supplied by the reactive primary power supply; and PA1 (3) all the above system characteristics resulting in: PA1 (a4) means of raising the d.c. supply voltage made available to power the system smoke alarms to its original quiescent value when a higher than quiescent current is detected for a period of time exceeding the duration of the self-test period; and PA1 the smoke alarms can further comprise: PA1 (b3) means by which the higher than quiescent current following an alarm being raised, after a short initial period exceeding the duration of the self-test period, is caused to be supplied exclusively to the reactive primary power supply for the remainder of the alarm period.
Type 2
A smoke detection and alarm system comprising of dual supply smoke alarms where the primary (normal) power supply is mains supply and the stand-by power supply is in the form of non-rechargeable batteries as found in Type 1 smoke alarm previously described.
In Type 2 smoke alarms, although the smoke detection functions of the smoke alarm are carried out using power from mains supply, the stand-by battery is constantly being monitored and periodically tested as described for Type 1 smoke alarms.
The difficulties/disadvantages of Type 2 smoke alarms are:
Type 3
A smoke detection and alarm system comprising of dual supply smoke alarms as for Type 2 except that the smoke alarms stand-by batteries are of the rechargeable type.
The difficulties/disadvantages of Type 3 smoke alarms are as previously described for Type 2 smoke alarms. Type 3 smoke alarms have the further disadvantage that the rechargeable batteries are relatively expensive and require the provision of a battery charging circuit which also adds to the overall cost of the system.
Type 4
A smoke detection and alarm system comprising of dual supply smoke alarms where the primary (normal) power supply is of the extra low voltage type derived from mains supply, and the stand-by power supply is in the form of a rechargeable battery which is normally part of a separate control box/panel.
The main difficulty/disadvantage with a system comprising of Type 4 smoke alarms is that, apart from the high cost of the rechargeable battery, the system also requires a battery charger and an electronic circuit to test the battery condition. The battery testing furthermore becomes a manual function of the system so that the overall reliability of the system is greatly reduced when considering the fact that the standby batteries of Types 1, 2 and 3 smoke alarms are automatically tested at least once every minute by the smoke alarm internal electronic circuitry.